The function of a papermachine headbox is to receive rawstock from the inlet manifold and to deliver the stock to the slice and forming region of a paper machine at the proper velocity, and in uniformly mixed condition so that there is a minimum of variation with respect to fiber distribution, consistency, volume and velocity.
As machine speeds and widths have increased, headbox design has become a factor of increasing importance. The old conventional open type box has given way to radically new designs. On high speed machines, the headboxes have become essentially hydraulic nozzles equipped with flow spreading devices which bring the stock in and distribute it at several points across the full width of the papermachine. The problems encountered in the design of a headbox for a high speed papermachine are in many ways similar to hydraulic problems in other industries. However, the problem of spreading paper making stock uniformly in a thin film across the width of a modern, high speed papermachine is a very difficult problem from the hydraulic standpoint alone, and when coupled with the additional demands of maintaining the suspended paper making fibers in the proper state of distribution, it becomes probably the most difficult and complex process in the whole paper making operation.
Most of the designs of modern flow systems are aimed at breaking up flocculation by increasing the intensity of turbulence and shear rate, followed by a reduction in the scale of turbulence. In the pressure type flow nozzle, perforated rolls, as well as cross current rectifiers are used to (1) create turbulence, and (2) even out velocity distribution or profile. If the intensity of turbulence at the rectifier roll or rolls is sufficiently high, flocs are broken up, and subsequently, the scale of this turbulence is then reduced by turbulence decay which follows the perforated roll.
Pressure pulsations in the approach piping to the headbox are not a factor in flocculation or fiber dispersion, but they do produce flow variations in the slice and show up as cyclic short term variations in basis weight of the paper in the machine direction. Thus it is extremely important to prevent pressure fluctuations at the slice if a uniform basis weight is to be obtained.